Process control apparatus and method

ABSTRACT

A process control apparatus and method in which an adjustable control signal is continuously applied to a process control transducer while components of the printed circuit control system are removed for repair or modification. By transferring to manual control by an output driver card, the controller card which provides the signals for the regulator mode of operation may be removed and replaced by a service card. A manual signal generated by the service card is matched with that of the driver card before transfer to the service card is effected and the driver card is removed. An automatic reset feature on the service card precludes inadvertent transfer as the card is plugged in. The manual setting on the driver card may be controlled from a remote station for process control purposes, or from the service card when installed for matching signals when the driver card is replaced. No extra plug-in units are required in the printed circuit rack since the service card replaces the controller card.

i United States Patent 1 Cook et al.

[ PROCESS CONTROL APPARATUS AND METHOD [75] Inventors: Henry F. Cook; James F.

Sutherland, both of Pittsburgh, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: July 5, 1972 21 Appl. No.: 268,953

[52] US. Cl 317/112, 317/101 DH, 200/46,

235/61.11 R, 340/147 SC [51] Int. Cl. H021) 1/04 [58] Field of Search 340/147 P, 147 SC,

340/151, 151.1; 235/6l.ll R; ZOO/46; 317/101 DH, 1 12 1 Oct. 30, 1973 Primary Exami nerRobert N. Schaefer Assistant ExaminerGerald P. Tolin Attorney-F. l-l. Henson et al.

57 ABSTRACT A process control apparatus and method in which an adjustable control signal is continuously applied to a process control transducer while components of the printed circuit control system are removed for repair or modification. By transferring to manual control by an output driver card, the controller card which provides the signals for the regulator mode of operation may be removed and replaced by a service card. A manual signal generated by the service card is matched with that of the driver card before transfer to the service card is effected and the driver card is removed. An automatic reset feature on the service card precludes inadvertent transfer as the card is plugged [56] References in. The manual setting on the driver card may be con- UNITED STATES PATENTS trolled from a remote station for process control pur- 3,483,543 12/1969 Flanagan 340/187 poses, or from the service card when installed for 3,364,467 1/1968 Haibl 147 SC matching signals when the driver card is replaced. No 3,573,558 4/197l 317/ 101 DH extra plug-in units are required in the printed circuit 3,566,355 2/1971 Smith 340/147 P rack since the service card replaces the controller card.

16 Claims, 3 Drawing Figures 48 /52 PROCESS 54 44 50 l 60 4o 56 PROCESS 1 46 CONTROLLER DRIVER CONTROL r I CARD L CARD /1 L TRANSDUCER 68 68-i Gal k T REMOTE STAT ION SERVlCE CARD ' 9 I I I I I I I PAIENIEDocT 30 ms SHEET 18F 2 PROCESS 44 PROCESS- CONTROL TRANSDUCER CONTROLLER CARD REMOTE STATION I PROCESS CONTROL APPARATUS AND METHOD BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to process control and particularly to apparatus and methods for maintaining continuous process control while components of the control system are removed from service for repairs.

2. Prior Art It is common practice to control industrial processes by applying electrical control signals generated by a controller to process control transducers which mechanically, hydraulically or pneumatically position final control elements such as valves, dampers, etc. These final control elements in turn regulate the flow of energy into or out of therprocess. Very often the final control element is biased-to either the full open or full closed position toprovide fail-safe operation thereby requiring that av continuous control signal be applied to the transducer to maintainthe finalcontrol element in any other desired. position. This requirement proves to be troublesome when it is desired to remove the'controller for servicing or'repair. It is particularly troublesome in controlling processeswhich cannot tolerate removal of control even for the. short interval required to replace the controller.

conventionally, industrial process controllers are provided with a regulator mode of operation wherein the control signals are generated as a function of a set point and a feedback signal from the process or the final control element, and a manual mode wherein the control signal is manually set to a desired level. In the past, it has been a practice to provide separate regulator and manual controllers each with its own power supply so that either one'could be removed or repaired in place while control was shifted to the other unit. However, in many modern process control systems which require numerous controllers to position many control elements each of which require relatively high level control signals, it is not practical to provide com pletely separate manual and. regulator type controllers for each final control element. For this reason, and the fact that electronic circuits are becoming more reliable, the trend is to combine-the. regulator and manual functions in a single unit having a common power supply.

Although the reliability of the electronic circuits has vastly improved, especially with the introduction of solid state components, it remains desirable'to provide the capability of removing a controller from service while maintaining, process control. U..S. Pat. No.

3,483,543 proposed that a by-pass unit in the form of ual control of the associatedtransducer. The control signal from the automatic controller is routed through the by-pass unit where it is compared with the signal developed by the by-pass unit beforethe transfer is made. This arrangement requires additional space in the instrument panel for the by-pass unit, and requires such additional space for each by-pass unit if the capability of removing more than one process controller at a time is to be provided. Furthermore, inadvertent transfer to a mismatched control signal can occur if the transfer switch is in the by-pass positionwhen the jack is installed.

SUMMARY OF THE INVENTION According to the invention, a continuous adjustable control signal is delivered to a process control transducer by a service unit while a printed circuitcontroller unit is removed for service. A variety of printed circuit cards which makev up the process controller, are plugged into plug-in units mounted in a printed circuit rack assembly. A first and second of the plug-in units are connected through a common output circuit to a process control transducer. A removable driver card which generates acontrol signal for the process control transducer is plugged into the first plug-in unit. The driver card may be operated either in a regulator mode wherein itgenerates the control signal as a function of an applied signal orin a manual mode wherein it generates a manually adjustable control signal.

A service card which also generates a manually adjustable control signal for the process control transducer can be plugged into the second plug-in unit in the printed circuit rack assembly. Transfer of control between the driver card and the service card is accomplished by transfer means including a first part on the driver card and a second part on the service card. The transfer means has a normal condition wherein the control signal from the driver card is connected to the output circuit and a service condition wherein the service card supplies the control signal to the process control transducer. The two parts of the transfer-means are interconnected through the first and second plug-in units with the control means mounted on the service card.

Signal level indicator means are provided on the service card so that the control signals can be matched before the transfer'is made. Preferably, a meter which is calibrated to indicate from 0-100 percent of the final controlelement span it utilized so that a single meter may be used for matching the control signals and manually setting the final control element to a desired position. Matching of the signals is accomplished by switching the meter back and forth between the two control signals and adjusting the outputs until no needle movement is noticed.

In order to avoid premature transfer to the service card before the control signals have. been matched, reset means are provided'which force the first and second parts of the transfer means to the normal condition when the service card is first plugged in. Preferably, this is accomplishedby the application of power to the service cardthrough the plug-in unit. Additional means on'the. service card force the driver card. to the manual mode of operation and permit manual adjustment of the driver card control signal from the service card.

Preferably, the service card is inserted into the plugin unit normally occupied by a controller card which supplies the applied signal to the driver card when it is operating in the regulator mode. This applied signal is generated'as a function of a desired set point and a processvariable feedback signal. By transferring the driver card to the manual mode, the controller card may be removed and replaced by the service card. With this arrangement, it is not necessary to provide extra space in the rack assembly for service cards. This is especially important in large control systems incorporating numerousprocess controllers, and permits removal of any number of process controllers at the same time.

The invention also embraces the method of continuously supplying a control signal to a process control transducer in a control system such as that described above, by transferring the driver card from the regulator mode to the manual mode, removing the controller card, replacing it with the service card, and then transferring control to the service card. The method further includes the step of matching the control signals from the driver card and the service card before the transfer is effected, preferably by alternately applying the control signals to a level indicator and manually adjusting the signal levels. It is also desirable that the level indicator be isolated from the control signal sources to avoid distortion of the readings.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustratesa'process control system in which a process controller identified generally by the reference character 40 generates a control signal which is applied through an output circuit 42 to a process control transducer 44. The process control transducer actuates the'final process control element 46 which regulates the flow of energy into or out of the process 48. Typically, thefinalcontrol element 46 may take the form of a valve which regulates the flow ofa fluid such as steam into or out of the processthrough piping 50. The valve 46 may be mechanically, hydraulically or pneumatically positioned by the process control transducer 44 in response to the control signal generated by the controller 40.

A feedback signal generated as a function of a measured process variable is applied through lead 52 to the controller 40. The controller 40 comprises a variety of printed circuit cards mounted in'a printed circuit card rack assembly 54. A driver card 56 generates the pro-. cess control signal which is applied to the output circuit 42 through the normally closed contacts 58 of a control a relay. The driver card 56 may be operated either in a regulator mode wherein it generates a control signal as a function of an applied signal," or a manual mode wherein it generates a manually adju table control signal. A controller card 60 generates the appliedsignal supplied through lead 52 and a set point. The set point is supplied to the controller card 60 from a remote station 62 which may be located in the plant control room. Transfer of the driver card between the regulator and manual modes of operation and adjustment of the manual control signal can be effected from the remote station 62.

In many control systems the final control element 46 is biased to the full-open or full-closed position to provide fail-safe operation. This necessitates that a control signal be continuously applied to the process control transducer to maintain the final control element in any other operating position. Since many processes will not tolerate the loss of control even for short intervals, alternate means must be made available for providing the continuous control signal when it is desired to remove the controller for servicing. To this end, a service card 64 is provided which generates a manually adjustable control signal which is applied to the output circuit 42 through the normally open contacts 66 of a control relay when the service card is plugged into the printed circuit rack assembly 54 through plug-in connectors 68. The service card is also plugged into the driver card so that the signals from the service card and the driver card may be matched before control is transferred to the service card and for other purposes which will be discussed hereafter. When the signals are matched, control is transferred to the service card by the closing of the contacts 66 and the opening of "the contacts 58 of the control relays. The driver card may then be unplugged and removed. Preferably, the service card is plugged into the space normally occupied by the controller card .60. This is accomplished by transferring the driver card to the manual mode of operation from the remote station 62 so-that the applied signal from the controller card is'no longer required, and then unplugging the controller card. The service card may then be inserted into the slot normally occupied by the controller card.

When a new driver card or the repaired card is reinserted in the rack assembly, itscontrol signal is manually adjusted to match that of the service card before control is returned to the driver card. With the driver card then operating in the manual mode, the service card can be removed and replaced by the controller card 60. The operator may then adjust the set'point applied to the controller card and transfer the driver card to regulator operation if desired. FIG. 2 illustrates how the printed circuit cards are mounted in the rack assembly. Each card is held in a slot formed by upper and lower channel members 70. When the card is fully inserted in the slot, a number of pin connectors in a connector strip 72 mounted on the rear edge of the card plug into plug-in units 74 of the rack assembly. Leads 75 connected to the plug-in units 74 interconnect the circuits on the various printed circuit boards and connect the boards with the input and output circuits of the rack assembly. The pin connectors on connector strip 72 are connected to the components (not shown) on the printed circuit board through ,the printed circuit. conductors (also not shown). The connectors may be arranged so that some of the pins make contact before others for purposes discussed switches 82 and 84, respectively, for adjusting the magnitude of the service card control signal, and the increase-decrease switch 86 for manually adjusting the control signal on the driver card.

Turning to FIG. 3, the driver card 56 includes a signal generator 88 which maybe operated in either the re gulator or manual mode under control of the .mode selector 90. In the regulatormode, the signal generator '88 generates a control signal .on lead 92 which is a function of a signal from the controller card which is applied through lead 94. In the manual mode, the signal generator 88 generates a control signal on lead 92 which may be increased or decreased by signals from the increase and decrease line receivers 96 and 98, respectively.

The signal appearing onlead 92 isapplied to the summing junction 100 of the operational amplifier 102 through resistor 104. "The other input to the amplifier 102 is connected to'ground through resistor 106. The output of theamplifier 102 is applied to thebase of the pnp transistor 108 through resistor 1 11. The emitter of transistor 108 is connected to .the base of another pnp transistor 110 which is provided with a heat sink as indicated by the dashed line encircling the transistor. The collectors. of transistors 108and 110 are connected to the negative terminal of a 40-volt D.C. supply- 112. The emitter of transistor 110 is connected through resistor 114 to ground and through feedback resistor 116.to the summing junction of amplifier 102. The operational amplifier 102 and the cascaded emitter-follower amplifier comprising transistors 108 and 110 convert the 2 to volt signal appearing on the lead 92 into a 4 to 20 milliamp output current through resistor 114. The

zener diode 118 connected between the base of transiswhen the card is removed. With the relay 122 deener gized, the normally closed contacts 58 connect the voltage-controlled current flowing through transistor 110 to pin 17 through over-voltage protection diode 126. Pins 17 and 22 connect the output of'the driver card to the process control transducer through output circuit 42. Pin 17 also cooperates with pin 19-to apply the output signal of the driver card to. the meter 62m in the remote station. 'With the relay 122'energized, contacts 58'open and the normally opencontacts'128 close toshunt the current generated by the driver card around the output circuit.

When the service card is inserted in place of the controller card, certain of the'pins on the service card are interconnected through the plug-in units to the corresponding pins on the driver card. For instance, the pins 11 and 12 and 15 and 16 on the service card are connected to the same pins of the driver card. Pin 15 on the service card is connected to the three-position IN- CREASE-DECREASE switch 86 which is spring loaded to the center off position. Pin 16 of the service card is connected to the other terminal of the switch. Pin 15 on the driver card is connected to the decrease line receiver 98 and pin .16 is connected to the increase line receiver 96. Thus, if the 'switch86 is movedto the lower position in FIG. 3, a decrease signal will be applied to the signal generator 88 on the driver card. Sim- I ilarly, an increase signal will be applied to the driver card signal generator if the switch is moved to the upper position. Power is supplied to the line receivers from a +5 volt supply on the service card through resistors 130 and 132 and the pins 11 and 12.The driver card manual signal may also be increased or decreased by signals from the remote station 62 as indicated.

With the service card in place, pins 6 and 9 on the two cards complete a circuit between the +5-volt supply and the ground on the service card through resistor 134 to force the mode selector 90-to transfer the driver card to manual control. This is particularly useful when the driver card is being replaced and it also prevents transfer from the remote station to the regulator'mode when the controller card has been removed to make room for the service card.

The service card signal generator includes a temperature compensated 6.2 volt reference voltagegenerator indicated generally by the reference character 136. The 6.2 reference voltage is developed across the zener diode 138 connected between a 20 volt supply .and ground. The remainder of the circuit provides the temperature compensation. The emitter of an npn transistor 140 is connected to the 20 volt supply through resistor 142. The collector of transistor 140 is connected through diode 144 and output junction 146 to the zener diode 138. A bias resistor 148 is connected between the base and the collector of transistor 140. This resistor also generates collector-to-base bias voltage for pnp transistor 150. The emitter of transistor 150 is connected through resistor 152 to ground. The collector of transistor 150 is connected to the base of transistor 140'and through diode 154 and zener diode 156 to the 20 volt supply. In operation, the zener voltage of zener diode 138 andthe forward drop of diode 144 cause a current of approximately 7.5 milliamperes to flow through resistor 152 by establishing the base drive voltage on transistor 150. This current flowing through the 6.2 volt zener 156 and diode 154 regulates the current through transistor 140 and, therefore, zener 138 to approximately 7.5 milliamperes. Temperature changes on the zener 138 will have a corresponding ef- 7 feet on the zener 156 to maintain the 7.5 milliampere current through zener 138.

The 6.2 volt reference voltage developed at junction 146 is appliedlto the summing junction 158 of the operational amplifier 160*through resistor 162. The other input to amplifier 160 is connected through resistor 164 to ground. The output voltage of the amplifier 160 is fed back to the-summing junction 158 through potentiometers 166 and 168 and resistor 170. By using a 20k potentiometer for 168 and a 1k potentiometer for 166, coarse and fine adjustments of the output voltage of amplifier 160 can be made. The potentiometers 168 and 166 are set by the thumb wheel switches .82 and 84, respectively, mounted on the leading edge of the service card. The amplifier 160 providesa 2 to 10 volt adjustable Ivoltage signal. This signal is applied through resistor .172 to the summing junction 174 of the operational amplifier 176. The other input of this amplifier is connected through the resistor 178 to ground. The operational amplifier 176 along with the cascaded emitter-follower amplifier comprising pnp transistors 180 and 182 and the-40 volt D.C. supply 184 comprise a voltage-controlled current source similar .to that described in connection with'the driver card above. The 2 to 10 volt signal applied "to the input of amplifier 176 is converted to a 4 to 20 milliampere curnected to one terminal of the meter switch 80 through resistor 190. Again a resistor 192 and zenerdiode 194 in the output of the operational amplifier limit the short circuit current signal. i

Normally, the control signal generated by the service card is short-circuited by the normally closed contacts 198 of the control relay indicated generally by the reference character 196. However, when this relay is energi'zed, the contacts 198 open and the normally open contacts 66 close to apply the control signalto pins 19- and 22 through diode 200 and to pin 17 through the additional diode 202. The pins 17 and 22 connect the service card control signal to the process control transducer 44 through the output circuit 42 of the printed circuit rack assembly. The pins 17 and 19 apply the output signal to the meter 62m in the remote station.

The coil of. the relay 196 on the service cardis connected inparallel with the coil of relay 122 on the driver card by the common pins 31 and 35. These relays are connected through resistors 204 and 206, respectively, between a +26 volt D.C. supply and the collector of an npn transistor 208. The emitter of transise tor 208 is connected to ground and'to its'own base through bias resistor 210. The transistor is protected by a diode 212 connected between its base and collector. Conduction by the transistor 208 is controlled by NAND 214 connected to the base of transistor 208 through resistor 216 and a diode 218 which protects the NA'ND. The NAND is a well-known logic element which develops a digital ONE signal at its output unless all of its inputs are digital ONE signals. Therefore, when a digital ONE signal is applied to the NAND element 214, its output will be ZERO to essentially ground the base of transistor 208- to cut the transistor off. When the input to NAND is ZERO, its output goes to a digital ONE which is approximately volts, to turn on transistor 208(Additionalbase drive current for transistor '208 can be provided from the +5 volt supply through resistor 220 to assist the NAND element 214.

The input to NAND element 214 is'derived from the upper output of the flip-flop designated generally as 222 comprising NAND 'elements' 224 and 226. The

- spectively, of themeter switch 80. Isolation is providedupper output of the flip-flop is also connected to NAND 228 while'the lower output is connectedto NAND 230. The outputs=of these NANDs are "connected through light emitting'diodes 232 and 234 in se-' ies with resistors 236 and238, respectively, to a +5 volt supply. The lower input to the flip-flop 222 is the lower terminal of the three-position transfer switch 76 which is spring loaded to the center off position. The upper terminal of this switch serves as an upper input to flip-flop 222.'Th'e common terminal of the switch is grounded. I

When the transfer switch 76 ismomentarily placed in the upper or NORMAL position a digital ZEROis applied to NAND 224 which forces the upper output of flip-flop 222 to ONE. The ONE applied to NAND 228 causes its output to go to ZERO to turn on the lightemitting diode 232 to indicate-that the transfercir 230. The ZERO signal at the upper output of flip-flop 224 turns off the NORMAL light-emitting diode 232 and turns on the transistor 208 through NAND 214. With the transistor 208 turned on, the relays 122 and 196 are energized by current flowing from the 26-volt supply through the coils of the relays and through the transistor 208 to ground. i

I In order to assure that the transfer circuitry is in the normal position when the service card is first inserted, the output of a NAND element 240 is also connected to the NAND 224 in the flip-flop. A second NAND 242 is connected to the input of NAND 240. The +5 volt supply voltage is applied directly to NAND element 240 but is applied to NAND element 242 through serially connected diodes 244 and 246. When the service card is first plugged in, the NAND element 240 will reach its operating voltage before NAND element 242. Since an open circuit on an input to a NAND element is registered as a ONE, the output of NAND element 240 will go to ZERO to force the upper output of flipflop 222 to go to'ONE, thereby forcing the transfer circuitry to the normal condition. Shortly thereafter, the voltage on NAND element242 will rise sufficiently to supply a ZERO to the input of NAND 240. The output of NAND 240 will, therefore, go to ONE so that-NAND element 224 may now be controlled by the input connected to the transfer switch.

In order to match the controlsignals on the driver card and the service card before the transfer is effected, the emitter voltages on outputtransistors 110 and 182 are applied to the meter 78 mounted on the service card through the upper and'lower terminals, re-

by applying the selected signal to the non-inverting input of operational amplifier 248 through the input resistor 250. A feed-back signal is applied to the other input of amplifier 248 through resistor 252. The high effective input impedance of an operational amplifier operated in this mode effectively isolates the applied signals. r

Suppression isapplied to the isolated signal so that the '4 to 20 milliamp control signal may'be converted to a O to -l0O percent reading on the meter 78. To this end, the isolated'signal is applied tothe summing'juncacross themeter dampensthe needle as the switch 80 is switched from one position to the otherduring matching of the control signals. A bias circuit, indicated generally by reference character 264, is connected to the summing junction of amplifier 254 to correct its output to zero for a 4 milliamp applied signal. The bias circuit includes the 6.2-volt zener diode 266 connected to a +15 volt supply through resistor 268. The bias may be adjusted by the potentiometer 270 in series with the resistor 272.

Assume that the driver and controller cards are installed and that the controller is operating in the regulator mode. Assume further that it is decided to remove the driver card. The first step is for the operator to transfer, the controller to the manual modefrom the rernote station. Open-loop control of the process may 9 ting signals to the increase or decrease line receivers from the remote station. With the driver card operating in the manual mode, the controller card maybe removed. The service cardmay then'be installed in the slot formerly occupied by the controller card. Insertion of the service card will interconnect the circuits on the driver card and service card as indicated above. The initial application of power to the service card will force the transfer flip-flop to the normal condition as will be indicated by illumination of the NORMAL lightemitting diode 232. This will also prevent'energization of the relays I22 and 196 so that the current signal generated by the driver card will remain connected to the process control transducer 44 through the normally closed contact 58 of relay 122. Initial insertion of the service card will also lock the driver card in the manual mode through the signal applied to the mode selector 90 through the pins 9 and 6.

The control signal developed by the-servicecard may be adjusted by the thumbwheel switches 82 an 84 on the front of the service card to match the signal being applied to the process control transducer 44 by the driver card. The signals are matched by adjusting the thumb wheel switches and then moving the meter switch 80 back and forth until no needle movement is observed on switching. In this manner, the signals may be matched within k percent.

Control is then switched to the service card by depressing the transfer switch 76 to the SERVICE'position. This reverses the outputs of flip-flop 222 to turn on the SERVICE light-emitting diode'234 and the transistor 208. With transistor 208 conducting, both the relays 122 and 196 are energized simultaneously. This opens the contacts 58 and closes the contacts 66 to remove the driver card control signal from the process control transducer 44 and replace it with the control signal from the service card. The driver card may then be removed and the process may be manually controlled by operation of the thumb wheel switches on the service card.

When the driver card is replaced, the signal transmitted by pins 6 and 9 will assure that it is forced to the manual mode. Since at the time that the driver cardis replaced the transistor 208 is conducting, the relay 122 will be picked up to open the contacts 58, thus preventing the random signal initially generated by the driver card from being applied to the process control transducer. The pins 31 and 35 which provide the energization for the relay 122 from-the service cardmake contact before the pins (not shown) which supply power to the driver card so that contacts 58 are opened before an output signal can be generated. This permits the signal from the driver card to be matched with the existing signal generated by the service card before transfer-tov the driver card is made. The signals are matched by utilizing the increase-decrease switch 86 on the service card to manually adjust the driver signal andoperating the meter switch back and forth until no difference be tween the two signals is observed. Transferto the driver card is then accomplished by operating the transfer switch 76 to the NORMAL-position. This will turn on the NORMAL light-emitting diode 232'and cut off the transistor 208, which in turn will de-energize relays 122 and 196 to close contacts '58 and open contacts 66. The service card may then be removed and replaced by the controller card. If desired, the controller may then be returned to the regulator mode of operation.

From the above discussion, it can be seen that the invention provides a simple convenient and space-saving manner of maintaining process control while portions of the controller are taken off line for service.

What is claimed is:

1. Process control apparatus including:

a printed circuit board rack assembly including first and second plug-in units and an output circuit connecting each of said plug-in units to a remotely located process control transducer,

a removable printed circuit driver card which plugs into the first plug-in unit, and which includes means for generating control signals for the process control transducer, said driver card'having a regulator mode of operation wherein it generates a con trol signal as a function of an applied signal and a manual mode of operation wherein it generates a -manually adjustable control signal,

a-removable printed circuit service card which plugs into said second plug-in unit, said service card having means'for generating a control signal for the process control transducer, and'manually adjustable means for controlling the magnitude of the control signal,

transfer means having a first part mounted on the driver card and operative between a normal condition wherein the control signal generated by the driver card is connected to the output circuit through the first plug-in unit, and aservice condition wherein the control signal is disconnected from the output circuit, a second part mounted on the service card and operative between a'normal condition wherein the signal generated'by the service card is disconnected from the output circuit and a service condition wherein the control signal generated by the service card is connected to the output circuit through the second plug-in unit, and control means mounted on the service card and connected directly to the second part of the transfer means and connected to the first part through the first and second plug-in units, said control meansbeing operative to simultaneously operate both the first and second parts of the transfer means between their normal and service conditions, whereby the driver card may be removed for service without interrupting the'application of a control signal to the process control transducer by transferring control to the service card.

2. The apparatus of claim 1 including meter means "mounted on the service card,and means for alternately connecting the control signal from the service card and, through the first and second plug-in units, the control signal from the driver card to the meter so that these two signals may be matched before transfer is effected.

3. The apparatus of claim 2 including means for elecitrically-isolating the meterfrom the control signals so advertently made before the signals can be matched.

5. The apparatus of claim 4 wherein the rack assembly includes a power supply connected to supply power to the printed circuit cards through said first and second plug-in units and wherein said reset means is activated by the application ofpower as the service card is plugged into the second plug-in unit.

6. The apparatus of claim 4 wherein the driver card is connected through the first plug-in unit to a remote control and monitoring station incorporating means for transferring the driver card between the regulator and manual modes of operation and for manually adjusting the magnitude of the control signal in the manual mode, said combination including means mounted on the service card and connected to the driver. card through the first and second plug-in units for manually adjusting the magnitude of the driver card control signal.

7. The apparatus of claim 6 including means mounted on the service card and connected to the driver card through the first and second plug-in units for holding the driver card in the manual mode of operation while the service card is plugged into thesecond plug-in unit, whereby the driver card cannot be inadvertently transferred to theregulator mode from the remote station while the service card is in place and whereby the driver card is forcedto the manual mode when it is replaced after servicing.

8. The apparatus of claim 1 including a controller card which plugs intosaid second plug-in unit in place of the service card under normal operating conditions and which includes means for generating said applied signal which determines the magnitude of the control signal generated by the driver card when it is operating in the regulator mode from a set point signal and a process variable feedback signal, said combination including means for connecting the applied signal to the driver card through said first and second plug-in units.

9. Process control apparatus including: a printed circuit board rack assembly including first r and second plug-in units and an output circuit connecting each of said plug-in units to a remotely located process control transducer, removable printed circuit driver card which plugs into said first plug-in unit and which; includes means for generating a control signal for said process control transducer, said driver card having a regulator mode of operation wherein it generates a control signal as a function of an applied signal and a manual mode of operationwherein it generates a manually adjustable control signal,

means for switching the driver card between the regulator and manual modes of operation,

a removable printed circuit controller card which plugs into said second plug-in unit and which gencrates said applied signal for the regulator mode of operation of the driver card from a set point signal and a process variable feedback signal, means connecting the applied signal from the eontroller card to the driver card through the first and second plug-in units,

a removable printed circuit service card which may transfer means operative between a normal condition wherein the control signal from the driver card is connected to the output circuit through the first plug-in unit and a service condition wherein the control signal from the service card when plugged into the secondplug-in unit in place of the controller card is connected to the output circuit through the second plug-in unit, whereby the driver card may be removed for service without the interruption of a control signal to the process control transducer by switching the driver card to the manual mode of operation, replacing the controller card with the service card and transferring control to the service card.

10. The apparatus of claim 9 including means mounted on the service card and connected to the driver card through the, first and second plug-inunits for comparing the control signals generated by the two cards so that they may be matched by the manualadjusting means before transfer is effected.

11. The apparatus of claim 9 including reset means operative to reset the transfer means to the normal condition when the service card is first plugged into the second plug-in unit.

12. A method of maintaining continuous control of a process control system which includes a printed circuit board rack assembly having two plug-in units each connected through a common output circuit to a process controltransducer, a removable printed circuit driver card plugged into a first of said plug-in units and having a regulator mode of operation wherein it generates a control signal for the process control transducer as a function of an applied signal and a manual mode of operation wherein it generates a manually adjustable control signal, a removable printed circuit controller card plugged into the second plug-in unit for generating said applied signal and a plug-in service card which generates a manually adjustable control signal, said method comprising the steps of:

transferring the driver card to the manual mode of operation wherein it supplies a manually adjustable control signal to the process control transducer through the output circuit connected to the first plug-in unit, removing the controller card from the second plug-in unit, plugging the service card into the second plug-in unit,

and simultaneously disconnecting the control signal generated by the driver card from the output circuit and connecting the control signal from the service card to the output circuit through said second plugin unit. 13. The method of claim 12 including the step of manually adjusting the level of the control signal generated by the service card to match the control signal on the driver card before switching from the driver card to the service card.

14. The method of claim 13 including the step of alternately applying the control signal from the driver card and the control signal from the service card to a signal'level indicator to determine when the two control signals are matched.

15. The method of claim 14 including the step of isolating the signal level indicator from the control signals whereby the step of applying the control signals to the signal level indicator does not affect the magnitude of the control signals.

16. The method of claim 13 including the final step of removing the driver card. 

1. Process control apparatus including: a printed circuit board rack assembly including first and second plug-in units and an output circuit connecting each of said plug-in units to a remotely located process control transducer, a removable printed circuit driver card which plugs into the first plug-in unit, and which includes means for generating control signals for the process control transducer, said driver card having a regulator mode of operation wherein it generates a control signal as a function of an applied signal and a manual mode of operation wherein it generates a manually adjustable control signal, a removable printed circuit service card which plugs into said second plug-in unit, said service card having means for generating a control signal for the process control transducer, and manually adjustable means for controlling the magnitude of the control signal, transfer means having a first part mounted on the driver card and operative between a normal condition wherein the control signal generated by the driver card is connected to the output circuit through the first plug-in unit, and a service condition wherein the control signal is disconnected from the output circuit, a second part mounted on the service card and operative between a normal condition wherein the signal generated by the service card is disconnected from the output circuit and a service condition wherein the control signal generated by the service card is connected to the output circuit through the second plug-in unit, and control means mounted on the service card and connected directly to the second part of the transfer means and connected to the first part through the first and second plug-in units, said control means being operative to simultaneously operate both the first and second parts of the transfer means between their normal and service conditions, whereby the driver card may be removed for service without interrupting the application of a control signal to the process control transducer by transferring control to the service card.
 2. The apparatus of claim 1 including meter means mounted on the service card, and means for alternately connecting the control signal from the service card and, through the first and second plug-in units, the control signal from the driver card to the meter so that these two signals may be matched before transfer is effected.
 3. The apparatus of claim 2 including means for electricAlly isolating the meter from the control signals so that application of the signals to the meter does not affect the magnitude of the signal applied to the process control transducer.
 4. The apparatus of claim 1 wherein the transfer control means includes reset means operative to force the control means to operate the first and second parts of the transfer means to their normal conditions when the service card is first plugged into said second plug-in unit, whereby transfer to the service card will not be inadvertently made before the signals can be matched.
 5. The apparatus of claim 4 wherein the rack assembly includes a power supply connected to supply power to the printed circuit cards through said first and second plug-in units and wherein said reset means is activated by the application of power as the service card is plugged into the second plug-in unit.
 6. The apparatus of claim 4 wherein the driver card is connected through the first plug-in unit to a remote control and monitoring station incorporating means for transferring the driver card between the regulator and manual modes of operation and for manually adjusting the magnitude of the control signal in the manual mode, said combination including means mounted on the service card and connected to the driver card through the first and second plug-in units for manually adjusting the magnitude of the driver card control signal.
 7. The apparatus of claim 6 including means mounted on the service card and connected to the driver card through the first and second plug-in units for holding the driver card in the manual mode of operation while the service card is plugged into the second plug-in unit, whereby the driver card cannot be inadvertently transferred to the regulator mode from the remote station while the service card is in place and whereby the driver card is forced to the manual mode when it is replaced after servicing.
 8. The apparatus of claim 1 including a controller card which plugs into said second plug-in unit in place of the service card under normal operating conditions and which includes means for generating said applied signal which determines the magnitude of the control signal generated by the driver card when it is operating in the regulator mode from a set point signal and a process variable feedback signal, said combination including means for connecting the applied signal to the driver card through said first and second plug-in units.
 9. Process control apparatus including: a printed circuit board rack assembly including first and second plug-in units and an output circuit connecting each of said plug-in units to a remotely located process control transducer, a removable printed circuit driver card which plugs into said first plug-in unit and which includes means for generating a control signal for said process control transducer, said driver card having a regulator mode of operation wherein it generates a control signal as a function of an applied signal and a manual mode of operation wherein it generates a manually adjustable control signal, means for switching the driver card between the regulator and manual modes of operation, a removable printed circuit controller card which plugs into said second plug-in unit and which generates said applied signal for the regulator mode of operation of the driver card from a set point signal and a process variable feedback signal, means connecting the applied signal from the controller card to the driver card through the first and second plug-in units, a removable printed circuit service card which may be selectively plugged into the second plug-in unit in place of the controller card, and which includes means for generating a control signal for the process control transducer and means for manually adjusting the magnitude of the control signal, and transfer means operative between a normal condition wherein the control signal from the driver card is connected to the output circuit through the first plug-in unit and a service condition wherein the control signal from the service card when plugged into the second plug-in unit in place of the controller card is connected to the output circuit through the second plug-in unit, whereby the driver card may be removed for service without the interruption of a control signal to the process control transducer by switching the driver card to the manual mode of operation, replacing the controller card with the service card and transferring control to the service card.
 10. The apparatus of claim 9 including means mounted on the service card and connected to the driver card through the first and second plug-in units for comparing the control signals generated by the two cards so that they may be matched by the manual adjusting means before transfer is effected.
 11. The apparatus of claim 9 including reset means operative to reset the transfer means to the normal condition when the service card is first plugged into the second plug-in unit.
 12. A method of maintaining continuous control of a process control system which includes a printed circuit board rack assembly having two plug-in units each connected through a common output circuit to a process control transducer, a removable printed circuit driver card plugged into a first of said plug-in units and having a regulator mode of operation wherein it generates a control signal for the process control transducer as a function of an applied signal and a manual mode of operation wherein it generates a manually adjustable control signal, a removable printed circuit controller card plugged into the second plug-in unit for generating said applied signal and a plug-in service card which generates a manually adjustable control signal, said method comprising the steps of: transferring the driver card to the manual mode of operation wherein it supplies a manually adjustable control signal to the process control transducer through the output circuit connected to the first plug-in unit, removing the controller card from the second plug-in unit, plugging the service card into the second plug-in unit, and simultaneously disconnecting the control signal generated by the driver card from the output circuit and connecting the control signal from the service card to the output circuit through said second plug-in unit.
 13. The method of claim 12 including the step of manually adjusting the level of the control signal generated by the service card to match the control signal on the driver card before switching from the driver card to the service card.
 14. The method of claim 13 including the step of alternately applying the control signal from the driver card and the control signal from the service card to a signal level indicator to determine when the two control signals are matched.
 15. The method of claim 14 including the step of isolating the signal level indicator from the control signals whereby the step of applying the control signals to the signal level indicator does not affect the magnitude of the control signals.
 16. The method of claim 13 including the final step of removing the driver card. 